def OnOpen(self, event):

            if self.contentNotSaved:
                if wx.MessageBox("Current content has not been saved! Proceed?", "Please confirm",
                                 wx.ICON_QUESTION | wx.YES_NO, self) == wx.NO:
                    return

            # otherwise ask the user what new file to open
            with wx.FileDialog(self, "Open XYZ file", wildcard="XYZ files (*.xyz)|*.xyz",
                               style=wx.FD_OPEN | wx.FD_FILE_MUST_EXIST) as fileDialog:

                if fileDialog.ShowModal() == wx.ID_CANCEL:
                    return     # the user changed their mind

                # Proceed loading the file chosen by the user
                pathname = fileDialog.GetPath()
                try:
                    with open(pathname, 'r') as file:
                        self.doLoadDataOrWhatever(file)
                except IOError:
                    wx.LogError("Cannot open file '%s'." % newfile)